In the technology of industrial measurements and automation, often used for the highly accurate measurement of process measured variables of media flowing in lines, especially pipelines, are inline measuring devices utilizing measuring transducers of vibration type. Such transducers have at least one measuring tube communicating with the pipeline conveying the medium and vibrating during operation. The construction, functioning and applications of such measuring transducers of vibration-type are described in detail in, among other places, U.S. Pat. Nos. 4,127,028, 4,524,610, 4,768,384, 4,793,191, 4,823,614, 5,253,533, 5,301,557, 5,610,342, 6,006,609, 6,047,457, 6,168,069, 6,314,820, 6,352,196, 6,397,685, 6,450,042, 6,487,917, 6,516,674, 6,519,828, 6,523,421, 6,598,281, 6,666,098, 6,698,644, 6,711,958, 6,769,163, 6,776,053, 6,807,866, 7,005,019, US-A 2005/0160787, WO-A 03/048693, or WO-A 05/050144.
Measuring transducers of vibration-type serve, as is known, in conjunction with a measuring device electronics connected therewith, for producing in the medium instantaneously conveyed in the at least one measuring tube, reaction forces appropriately corresponding with the process measured variable to be measured, for instance Coriolis forces corresponding with a mass flow, inertial forces corresponding with a density, or frictional forces corresponding with a viscosity, and for producing, derived from these forces, a measurement signal appropriately corresponding with the process measured variable, for example the particular mass flow, viscosity and/or density. The at least one measuring tube of the measuring transducer is, for such purpose, usually installed in a manner tight to the medium, especially pressure tightly, and most often, also permanently, into the course of the pipeline conveying the medium, for example, by means of appropriate flange connections. For the oscillatable holding of the at least one measuring tube, there is additionally provided a tubular, or frame-shaped, support element, for example one of steel, which is, in comparison to the measuring tube, most often very bending-stiff. The support element is mechanically coupled to the measuring tube, or tubes, for example directly affixed thereto, on the inlet and outlet sides. The support element can, as is usual in the case of measuring transducers of such type and also directly perceivable from the above-cited state of the art, supplement the already mentioned transducer housing as appropriately externally applied covers, for instance by means of tubularly covering caps or laterally attached sheet or plate, or the support element can itself be built as the transducer housing.
For driving the at least one measuring tube, measuring transducers of the described kind additionally include an exciter mechanism electrically connected with the measuring device electronics and having an oscillation exciter, especially an electrodynamic or electromagnetic oscillation exciter, mechanically interacting with the measuring tube. During operation, the exciter mechanism is so activated in suitable manner by the measuring device electronics by means of corresponding exciter signals, that the measuring tube executes, at least temporarily, vibrations, especially bending oscillations and/or torsional oscillations. Furthermore, a sensor arrangement is provided for producing oscillation measurement signals. The sensor arrangement has, at least in the case of use of the measuring transducer as a Coriolis mass-flow measuring transducer, at least two mutually spaced sensor elements reacting to inlet-side, respectively outlet-side, vibrations of the measuring tube.
Besides the possibility of simultaneous measurement of a plurality of such process variables of flowing media, especially mass flow, density, and/or viscosity, by means of one and the same measuring device, an additional, significant advantage of inline measuring devices with measuring transducers of vibration type is, among other things, that they have, within predetermined operating limits, a very high accuracy of measurement, coupled with relatively small sensitivity to disturbances. Beyond this, such a measuring device can be used for practically any flowable medium and can be installed in a variety of different areas of application in the technology of measurements and automation.
In the case of inline measuring devices of the described kind, which are used as Coriolis mass-flow meters, the measuring device electronics included as a part thereof measures, during operation, among other things, a phase difference between the two oscillation measurement signals delivered by the sensor elements and issues at its output a measured value signal derived therefrom. This measured value signal represents, as a function of time, a measured value corresponding to the mass-flow. If, as usual in the case of such inline measuring devices, also the density of the medium is to be measured, then the measuring device electronics measures therefor, on the basis of the oscillation measurement signals, additionally, an instantaneous oscillation frequency of the measuring tubes. Moreover, also, for example, the viscosity of the medium can be ascertained on the basis of the power, especially a corresponding exciter current for the exciter mechanism, needed for maintaining the measuring tube oscillations.
For operating the measuring transducer, especially also for the further processing or evaluation of the at least one measurement signal, such is, as already indicated, electrically connected with a corresponding measuring device electronics. In the technology of industrial measurements and automation, this measuring device electronics is additionally often connected via an attached data transmission system, with other measuring devices and/or with a remote central-computer, to which it sends, e.g. via a digital data bus, the measured value signals. Serving, in such case, as data transmission systems are often bus systems, especially serial bus systems, such as e.g. PROFIBUS-PA, FOUNDATION FIELDBUS, as well as the corresponding transmission protocols. By means of the central computer, the transmitted measured value signals can be further processed and visualized as corresponding measurement results e.g. on monitors and/or converted into control signals for corresponding control elements, such as e.g. solenoid-operated valves, electric motors of pumps, etc. For accommodating the measuring device electronics, such inline measuring devices further include an electronics housing, which can, as proposed e.g. in WO-A 00/36379, be located remotely from the measuring transducer and connected therewith only via a flexible line, or which, as shown e.g. also in EP-A 1 296 128 or WO 02/099363, is arranged directly on the measuring transducer, especially on a measuring transducer housing, housing the measuring transducer.
In the case of measuring transducers of the described kind, essentially two types of tube shapes have established themselves in the market, namely, on the one hand, essentially straight measuring tubes, and, on the other hand, measuring tubes essentially curving in a tube plane, among these being tubes of essentially S-, U- or V-shape, such as are most often used. Especially in the case of Coriolis mass-flow measuring transducers serving for measuring mass flows, in the case of both types of tube shapes, for reasons of symmetry, most often two measuring tubes are used. These two measuring tubes extend, at rest, essentially parallel to one another and are, most often, also flowed-through in parallel by the medium. In this connection, reference can be made, by way of example, to U.S. Pat. Nos. 4,127,028, 4,768,384, 4,793,191, 5,301,557, 5,610,342, 5,796,011, or 6,450,042.
Besides measuring transducers with such a double measuring tube arrangement, there are, however, also measuring transducers, available for a long time in the market, that are characterized by a single, straight or curved, measuring tube. Such measuring transducers of vibration-type having a single measuring tube are described e.g. in U.S. Pat. Nos. 4,524,610, 4,823,614, 5,253,533, 6,006,609, 6,047,457, 6,168,069, 6,314,820, 6,397,685, 6,487,917, 6,516,674, 6,666,098, 6,698,644, 6,711,958, 6,807,866, WO-A 03/048693, or WO-A 05/050,144. Each of the measuring transducers disclosed therein includes, among other things, a measuring tube having an inlet end and an outlet end, vibrating at least at times, and built, for example, of steel, titanium, tantalum or zirconium, or suitable alloys with one or more of these metals, for conveying the medium to be measured.
For the aforementioned case, in which the measuring transducer is one with a single measuring tube, there is provided in the measuring transducer, additionally, a counteroscillator, which is affixed to the measuring tube, especially a counteroscillator which is mounted oscillatably in the measuring transducer housing. Besides functioning for support of the oscillation exciter and sensor elements, the counteroscillator serves for decoupling, as regards oscillations, the vibrating measuring tube from the connected pipeline. The counteroscillator, which is most often made of cost-favorable steel and serves practically also as an internal support element, can, in such case, be embodied e.g. as a tubular compensation cylinder or box-shaped support frame arranged coaxially with the measuring tube.
The measuring transducers of vibration-type used in industrial measurements and automation technology face very high demands as regards their accuracy of measurement, which lies usually in the range of about 0.1% of the measured value and/or 0.01% of full scale. To achieve this, especially, a very high stability of the zero point is required, as well as also a very high robustness of the delivered measurement signals, especially also in the face of environmental, seating and/or operating conditions, which can change significantly during operation. As already discussed in detail in the mentioned U.S. Pat. Nos.5,610,342, 6,047,457, 6,168,069, 6,519,828, 6,598,281, 6,698,644, 6,769,163, WO-A 03/048693, or WO-A 05/050144, for such, especially important is the mechanical strength, especially the fatigue strength, of the connection of the at least one measuring tube to the at least one support element. Apart from the fact that the operational safety of the entire measuring device can depend thereon, already the smallest deviation of the strength of this connection from that which existed at the time of the calibration can lead also to significant, no longer manageable fluctuations of the zero point and, consequently to essentially unusable measurement signals. Usually, such zero-point errors attributable to loss of strength in the measuring transducer can only be satisfactorily overcome be installation of a new inline measuring device.
As already discussed in detail in U.S. Pat. Nos. 5,610,342, 6,047,457, 6,168,069, 6,598,281, 6,634,241, or WO-A 03/048693, especially the joining technology applied for the securement of the measuring tube within the outer support element and to the possibly also present counteroscillator has a considerable influence on the accuracy of measurement in general and on the stability of the zero-point. Traditionally, the measuring tubes and support element are at least partially affixed to one another on the basis of material bonds brought about by soldering, brazing and/or welding. Thus, for example, already in U.S. Pat. No. 4,823,614, it is described, that the respective end of the one measuring tube is inserted into a respective bore of an inlet-side, respectively outlet side, endpiece of the support element and affixed therein by frontside or rearside welding, soldering or brazing; compare the material beads to be seen in some of the figures. The endpieces are, in turn, affixed in a jacketing tube of the outer support element. Further examples of such materially bonded connections of measuring tube and support element are shown, in among others, also U.S. Pat. Nos. 6,168,069, 6,352,196, 6,519,828, 6,523,421, 6,598,281, 6,698,644, or 6,769,163.
As described in U.S. Pat. No. 5,610,342, if the heat supplied to the affixing locations of the measuring tubes with the endpieces during the mentioned welding, soldering or brazing leaves behind, following cooling, a state of mechanical, residual stress, such can lead to stress corrosion cracking, which then more or less weakens the joints and/or the measuring tube material in this way also. A further problem of such materially bonded, weld, solder or braze connections is mentioned in U.S. Pat. No. 6,519,828 or 6,598,281, this being material-wearing, oscillatory rubbing in the areas of the joints. Moreover, as perceivable from U.S. Pat. Nos. 6,047,457, 6,168,069, 6,352,196, 6,598,281, 6,634,241, 6,523,421, or 6,698,644, problems can arise relative to the permanence of soldered connections, problems which can be traced back to, among other things, insufficient wetting and/or radially alternating, mechanical stressing of the joints. As a result, often a decrease in the nominal pull-out strength of the measuring tube out of the support element is present. Additionally, in the case of heat-treated measuring tubes, changes extending into the depth of the material itself are present, be it with respect to the microstructure or the chemical composition. Accompanying this are significant changes of the material parameters, such as e.g. modulus of elasticity, ultimate tensile strength, ductility, etc., relevant for the oscillatory characteristics, as well as also for the component strength of the measuring transducer. This can especially degrade the opportunities for use of measuring transducers of the described kind in areas of application experiencing extremely high or low temperatures of the medium and/or significant temperature fluctuations, as well as also in areas of application experiencing extremely high media pressures of far above 500 bar.
For improving the long-term strength of measuring transducers of the described kind, the already mentioned U.S. Pat. No. 5,610,342, as well as also WO-A 03/048693, propose a securement method for measuring tubes in endpieces of the support element, wherein each end of the measuring tube is inserted into a corresponding bore of an inlet-side, respectively outlet-side, endpiece and pressed against the inner wall of the bore, especially without the application of heat, by means of a roller tool placed in the end, whereby a high-strength, frictionally interlocking connection is formed between the two components. A roller tool suitable for this method is described, for example, also in U.S. Pat. No. 4,090,382 in the context of a method for manufacturing boilers and heat exchangers.
A further possibility for manufacture of such connections between measuring tube and support element formed by means high-strength, frictionally interlocking is, as e.g. also proposed in U.S. Pat. No. 6,047,457, to externally compress the endpiece, following its insertion, or pushing, onto the measuring tube, by means of a pressing tool, and, at the same, to deform in a mixed plastic-elastic mode, beneath a recrystallization temperature of the material of the endpiece, especially at room temperature. The deformation forces used therefor are, in such case, and in each case, so developed, that the measuring tube experiences essentially no cross-sectional tapering and/or narrowing, so that an initial inner diameter of the measuring tube remains, following the securement, essentially unchanged all the way through. An apparatus correspondingly suited for the pressing is shown, for example, in U.S. Pat. No. 3,745,633. Alternatively or in supplementation of the plastic-lastic pressing, such a frictional interlocking can also be produced, as shown e.g. in U.S. Pat. No. 6,598,281 or U.S. Pat. No. 6,519,828, by thermally shrinking a corresponding metal body, be it the aforementioned endpiece or a metal sleeve placed on the measuring tube, etc., or, as also shown in WO-A 05/050144, by clamping the endpiece with the measuring tube using an interposed, elastically deformable clamping element.
Going further, it is discussed in U.S. Pat. No. 6,598,281 or U.S. Pat. No. 6,519,828, that also in the case of purely frictionally interlocking, press connections, due to oscillatory rubbing, it is not always possible to prevent, with certainty, a possible strength loss of the joined system. Moreover, such oscillatory rubbing can cause corrosion of the materials of the joined system in the region of the mutually contacting surfaces. Additionally, as perceivable from WO-A 03/048693, the usually different expansion behavior of the mentioned endpieces and the tube segments of the measuring tube in each case held therein, can lead, in the case of temperature fluctuations, especially in the case of possible temperature shocks, such as can arise e.g. in the case of regularly performed cleaning procedures with extremely hot washing liquids, to a sinking of the clamping forces exerted by the endpiece on the measuring tube below a critical value. This can mean, in turn, that the endpiece and the measuring tube, due to thermally caused expansions, lose, at locations, the mechanical contact brought about by the rolling, pressing or shrinking and so the press connection can be degraded to an unpermissible extent. As a result of this, in turn, the pull-out strength of the measuring tube out of the respective endpiece can sink and, therewith, also the required high zero-point stability of the measuring transducer achieved with press-joined assemblies can no longer be absolutely assured. For removing these deficiencies in measuring transducers of the described kind caused by oscillatory rubbing between measuring tube and respective endpieces, it is proposed in U.S. Pat. No. 6,598,281, respectively U.S. Pat. No. 6,519,828, also to weld the pertinent components together, following manufacture of the pressed assembly, especially in the case of use of a filling material serving as an intermediate layer, a practice which, however, possibly can raise anew the aforementioned problems associated with welded connections. In contrast, it is proposed in WO-A 03/048693 to achieve an increased anti-twist strength of the measuring tube and endpiece combination by forming a groove in the inner wall of the endpiece extending in the direction of the longitudinal axis of the joined system, in order to bring about a shape-interlocking effective in a circumferential direction for effectively preventing twisting of the measuring tube relative to the endpiece. However, even this connection can, especially in the case of use in a measuring transducer with a measuring tube executing, at least at times, bending oscillations, experience a lessening of the nominal pull-out strength, be it from oscillatory rubbing and/or from thermally related expansion.
Besides stability of the zero-point, also the sensitivity of the measuring transducer has a considerable influence on the accuracy with which a process measured variable is measured. In spite of the fact that measuring device electronics of inline measuring devices of the described kind are becoming ever more powerful and, as a result, always more precise, it is nevertheless to be acknowledged in this connection that, as regards accuracy of measurement, still an immense importance lies with the mechanical sensitivity of the measuring transducer. Mechanical sensitivity, in turn, depends on, besides the length of the measuring tube, also, to a very high degree, the ratio of the inner diameter of the measuring tube to its wall thickness. However, on the other hand, the material of the measuring tube can, in the case of a wall thickness which is too small for the operating pressure, be locally stressed to over its yield strength, a happening which, in turn, would mean a reduction of the ultimate tensile strength of the measuring tube as a whole.
In the dimensioning of measuring transducers of the described kind, a conflict thus results in the respect that, on the one hand, a high strength is to be secured for the measuring tube and, consequently, depending on the selected material, a correspondingly large measuring-tube wall-thickness is to be selected. On the other hand, however, the wall thickness is to be kept as small as possible, because of the required high measurement sensitivity. This has the result that measuring transducers of the described kind have been recommended over the years predominantly for applications with low or mid-range operating pressures up to about 400 bar or below. Measuring transducers for applications with high operating pressures of far above 500 bar, such as are found, for example, in applications with compressed hydrogen or other highly compressed gases, have been able to be recommended commercially, over the years, at best, only in the case of very small nominal diameters of less than 10 mm; these measuring transducers are, additionally, very expensive.